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Regulated Chloroplast Transcription Termination BBA - Bioenergetics 1860 (2019) 69–77 Contents lists available at ScienceDirect BBA - Bioenergetics journal homepage: www.elsevier.com/locate/bbabio Review Regulated chloroplast transcription termination T ⁎ ⁎⁎ Daili Jia, Nikolay Manavskib, Jörg Meurerc, Lixin Zhanga,d, , Wei Chia, a Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China b Centre National de la Recherche Scientifique (CNRS), Institut de Biologie Moleculaire des Plantes, 12 rue du General Zimmer, 67084 Strasbourg, France c Plant Molecular Biology, Faculty of Biology, Ludwig-Maximilians-University Munich, D-82152 Planegg-Martinsried, Germany d University of Chinese Academy of Sciences, Beijing 100049, China ARTICLE INFO ABSTRACT Keywords: Transcription termination by the RNA polymerase (RNAP) is a fundamental step of gene expression that involves Transcription termination the release of the nascent transcript and dissociation of the RNAP from the DNA template. However, the Chloroplast functional importance of termination extends beyond the mere definition of the gene borders. Chloroplasts Transcription originate from cyanobacteria and possess their own gene expression system. Plastids have a unique hybrid Rho factor transcription system consisting of two different types of RNAPs of dissimilar phylogenetic origin together with MTERF several additional nuclear encoded components. Although the basic components involved in chloroplast tran- scription have been identified, little attention has been paid to the chloroplast transcription termination. Recent identification and functional characterization of novel factors in regulating transcription termination in Arabidopsis chloroplasts via genetic and biochemical approaches have provided insights into the mechanisms and significance of transcription termination in chloroplast gene expression. This review provides an overview of the current knowledge of the transcription termination in chloroplasts. 1. Introduction type nucleus-encoded plastid RNA polymerase (NEP) and a prokaryotic type, plastid-encoded RNA polymerase (PEP) [9]. The PEP complex Viridiplantae possess two endosymbiotic organelles, chloroplasts comprises four core subunits α, β, β′, and β″, which display high si- and mitochondria, in contrast to metazoa and fungi harboring only the milarities to counterparts in cyanobacteria. Beside the “eubacterial” latter. These organelles originated from cyanobacteria and alpha-pro- subunits, a number of additional nucleus-encoded proteins of eu- teobacteria, respectively [1,2]. Although both organelles contain their karyotic origin involved in the chloroplast transcription have been own genomes, most endosymbiotic genes have been transferred to the identified in the PEP complex, the plastid TRANSCRIPTIONALLY AC- nucleus or got lost over the course of genome-organelle co-evolution TIVE CHROMOSOME (TAC) or the chloroplast nucleoid via proteomic [3,4]. This has resulted in highly reduced organellar genomes that re- approaches [10–15]. The genetic data have shown that many of those tained only a small number of the original genes. Most of the organellar play important roles in the accumulation of PEP-depended mRNAs proteins are encoded by nuclear genomes, synthesized in the cytosol [15–17] and tRNAs [18]. It is assumed that they might provide addi- and subsequently imported into the organelles. Thus, the proper ex- tional regulatory functions that adapt chloroplast transcription in re- pression of genes requires the tight coordination between the organellar sponse to environmental signals and developmental cues, however, the and nuclear genomes [5–7]. molecular mechanisms still remain largely unknown. Chloroplast gene expression is rather complex. It combines both Termination is the last important step of transcriptional processes eubacterial and eukaryotic features derived from the cyanobacteria [19]. The role of transcription termination is not restricted to the re- ancestor or the host cell [6]. However, many features especially of the lease of the RNAP from the DNA template. The growing evidence in- organellar gene expression system also evolved de novo [8]. One ex- dicates that it is also important to avoid interference with expression of ample is the chloroplast transcription machinery. The transcription of downstream genes, to prevent formation of antisense RNAs and to en- chloroplast genes depends on two RNA polymerases (RNAPs): a phage- sure a pool of RNAPs available for reinitiation or new transcription Abbreviations: RNAP, RNA polymerase; NEP, nucleus-encoded plastid RNA polymerase; PEP, plastid-encoded RNA polymerase; rut, Rho using termination; RNE, RNase E ⁎ Correspondence to: L. Zhang, University of Chinese Academy of Sciences, Beijing 100049, China. ⁎⁎ Correspondence to: W. Chi, Photosynthesis Research Center, Institute of Botany, Chinese Academy of Sciences, Nanxincun 20, Xiangshan, Beijing 100093, China. E-mail addresses: [email protected] (L. Zhang), [email protected] (W. Chi). https://doi.org/10.1016/j.bbabio.2018.11.011 Received 8 March 2018; Received in revised form 15 October 2018; Accepted 7 November 2018 Available online 08 November 2018 0005-2728/ © 2018 Elsevier B.V. All rights reserved. D. Ji et al. BBA - Bioenergetics 1860 (2019) 69–77 [20–22]. Termination mechanisms vary considerably in different or- resulted from transcription termination. The other half of the atpB 3′ ganisms, ranging from relatively simple to exceptionally complex pro- termini were produced by posttranscriptional processing events: in the cesses. In bacteria such as Escherichia coli, there are at least two con- first step the long atpB precursor transcript is endonucleolytically served mechanisms of transcription termination: Rho-dependent cleaved and then further processed by 3′-5′ exonucleolytic digestion to termination and intrinsic termination (dissociation of the transcription produce the mature transcript [45,46]. In addition, when the native 3′ complexes is accomplished without the assistance of Rho factors) end of petD was replaced by a variety of 3′ ends from other chloroplast [23–25]. In yeast and mammal cells, three different strategies for genes in Chlamydomonas, none of the 3′ ends in either sense or antisense transcription termination have been developed depending on distinct orientation prevented read-through, indicating that the 3′ end se- RNAPs [26]: termination by RNAP II is coupled with the processing of quences of chloroplast genes are not efficient transcription terminators the pre-mRNA [27–29], RNAP III termination is an autonomous process [47]. Therefore, it has been proposed that most of the RNA stem loop occurring at T-rich sequences located at the 3′-end of the genes [30,31], structures formed at the 3′ ends of mature RNAs in Chlamydomonas and RNAP I relies on a specific terminator composed of an oligo-dT chloroplasts might not be related to transcription termination. stretch and associated factors [32,33]. Interestingly, it was found that two tRNA genes, trnH1 and trnS, can Intriguing issues regarding the chloroplast transcription system are efficiently terminate transcription in a spinach in vitro transcription the mechanisms and significance of transcription termination. Due to system [43,48]. Furthermore, tRNA genes are often found downstream the cyanobacterial origin, one might expect that the termination ma- of several transcription units in the chloroplast genome of plants (e.g., chinery in chloroplasts is similar to that of bacteria. However, no atpBE, rrnV1; psaAB; tmfM; atpHFA, tmS3; psbA, tmH1)[49]. Thus, it homologs of known bacterial proteins involved in transcription termi- seems as if different vascular plant chloroplast tRNA genes might have a nation have been identified in chloroplasts so far. In addition, it is potential role in both termination and punctuation [50], which was generally accepted that it is the RNA maturation rather than the tran- originally established in vertebrate mitochondrial. The vertebrates scription termination that creates defined 3′ termini [34,35]. Thus, the mitochondrial genome is transcribed symmetrically as polycistronic degree to which the transcription termination affects chloroplast gene precursors spanning the entire heavy and light strands [51], however, expression remains an open question. For a long time, the termination the 22 tRNAs interspersed throughout the mitochondrial genome serve of transcription in chloroplasts has received relatively little attention as punctuation marks that are recognized and cleaved at 5′ and 3′ ends until two publications appeared that provided insights into the reg- by the mitochondrial nucleases to be process later on into mature ulation of chloroplast transcription termination in Arabidopsis [36,37]. species [50,52]. However, this concept does not hold true for Chlamy- In this short review, we will summarize the progress that has been made domonas chloroplasts [45,46]. in terms of the mechanism of chloroplast transcription termination, Despite these considerations, it is generally thought that the 3′ in- focusing especially on the functions of RHON1 and mTERF6, and pro- verted repeat sequences of chloroplast genes do not represent efficient vide perspectives in this area. Although some knowledge of chloroplast termination signals [45– 47]. However, their role in termination activity transcription termination arises from that of bacteria, mechanisms of could not be ruled out completely based on current evidences in spi- transcription termination
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